Individual and collaborative interdisciplinary experiments will be carried out on old rats and old cats, particularly in regard to the analysis of the auditory and sensorimotor cortices and the spinal cord. Immunochemistry of these neuronal populations will be studied and Golgi and electron microscopic descriptions will also be provided. Electrophysiological techniques will be used to study auditory evoked responses, spinal cord motoneuron membrane and cortical sensorimotor neuron potentials, and the histological correlates of the aging process in these regions will be examined.